1. Field of the Invention
The present invention relates to an artifical lightweight aggregate, and a process for producing the same.
2. Description of Prior Arts
The consumption of coal has greatly increased with increase of the use of coal as an energy source in the recent years. As a result, there has been proposed a problem in the disposal of coal ash formed thereby, and hence, it is greatly desired to effectively utilize the coal ash.
There has recently been a tendency that structures such as buildings are made more weight-saving and heat-insulating from the viewpoints of energy-saving and resource-saving and the viewpoint of improving the earthquake-proofness. Accordingly, an artificial lightweight aggregate (hereinafter, sometimes, referred to as ALA) of excellent quality appropriately employable for manufacturing lightweight concrete articles and a process for inexpensively producing the improving lightweight aggregate has been eagerly demanded.
Heretofore, various processes for producing ALA form coal ash as a main raw material have been proposed. These processes are disclosed, for instance, in Japanese Patent Publication Nos. 36(1961)-12580, 38(1963)-25820, 40(1965)-16270, 41(1966)-8239 and 47(1972)-47572, and Japanese Patent Provisional Publication No. 57(1982)-11866. However, the production of ALA from coal ash as a main raw material in Japan has not been industrially done as yet, mainly because there are disadvantages that a coal ash ALA produced by the above-mentioned known process is not economical in its manufacturing and inferior in quality as compared with expanded shale ALA commercially available in Japan (hereinafter referred to as commercially available ALA).
Certain comfirmative experiments of the processes for the production of ALA from coal ash which are disclosed in the aforementioned Patent Publications have been made by the present inventors. As a result, it has been found that as compared with the producing process of the commercially available ALA, the above-described processes have disadvantages in that a large amount of one or more auxiliary materials such as clay, shale, debris, feldspar, pulp waste liquor, magnesia and water glass must be added (uses of these auxiliary material are not economical), and a large amount of water must be used in granulating the raw material containing the auxiliary material so that an extra amount of fuel is comsumed for drying and hence, the cost of fuel is high. Further, it has been found that the ALA produced from coal ash by the known method has a low strength as an aggregate and a high water absorption so that the ALA is inferior in quality. Further, even if a lightweight aggregate of good quality could be obtained on a laboratory scale by the above-described processes, they had much difficulty in stably producing the aggregate on industrial scale and hence, there is a problem in industrialization.